1. Field of the Invention
The present invention relates to a parallax barrier, for instance for use in autostereoscopic three dimensional (3D) imaging such as 3D pictures and 3D displays. The present invention also relates to such 3D pictures and 3D displays. Applications include consumer and professional photography and display, 3D television, police identification, medical imaging, scientific visualisation and point of sales counters.
2. Description of the Related Art
FIG. 1 of the accompanying drawings illustrates diagrammatically an autostereoscopic 3D display of the front parallax barrier type. The display comprises a spatial light modulator (SLM) 1 in the form of a liquid crystal device (LCD). The SLM 1 is shown in simplified form as comprising substrates 2 and 3 containing a liquid crystal layer 4. Alignment layers, electrodes and other such elements are not shown in FIG. 1. The liquid crystal layer forms picture elements (pixels) such as 5, each of which is independently controllable to vary the transmission of light from a backlight (not shown). A parallax barrier 6 is provided on the front surface of the substrate 3. The parallax barrier 6 comprises an opaque layer in which are formed transparent apertures such as 7. The apertures 7 are parallel and evenly spaced and extend vertically. The display shown in FIG. 1 is of the type which supplies two two dimensional (2D) views forming an autostereoscopic pair so that each of the apertures 7 is aligned with and cooperates with two columns of pixels 5. The horizontal pitch of the apertures 7 is slightly less than twice the horizontal pitch of the columns of pixels 5 so as to provide viewpoint correction.
FIG. 1 illustrates how a viewing window 8 is formed at the intended viewing distance from the display. The apertures 7 of the parallax barrier 6 restrict the visibility of the pixels at the viewing plane, which contains the viewing window 8 and is located at the intended viewing distance from the display. The viewing window 8 represents the region at the viewing plane throughout which the pixel 5 is visible through the associated aperture of the parallax barrier 6. Two columns of pixels 5 cooperate with each aperture 7 to produce two adjacent viewing windows at the viewing plane. One of the columns of pixels associated with each aperture displays a vertical slice of one of the 2D images whereas the other column displays a slice of the other 2D image. When an observer is disposed so that the left and right eyes are at the corresponding viewing windows, the observer perceives a 3D image without having to wear any viewing aids.
FIG. 2 of the accompanying drawings illustrates diagrammatically how a display 10 of the type shown in FIG. 1 generates viewing zones 11 and 12 for the left and right eyes, respectively, of an observer. An arrow 13 represents the viewing plane at the intended viewing distance from the display 10. Because of the viewpoint correction, the viewing windows formed by the columns of pixels 5 and the apertures 7 across the surface of the display 10 are formed at the plane 13 and represent the widest parts of the viewing zones 11 and 12. When the eyes of the observer are located at the viewing windows in the viewing plane 13, the observer has the greatest lateral freedom of movement while being able to observe a 3D image. However, the viewing zones 11 and 12 illustrate that the observer also has some longitudinal viewing freedom. In other words, provided the eyes of the observer are located in the viewing zones 11 and 12, a 3D image will be observed.
In an ideal display of the type shown in FIGS. 1 and 2, the intensity distribution of light across each viewing window would be a xe2x80x9ctop hatxe2x80x9d function. In other words, for each viewing window, the light intensity would be constant across the viewing window and zero outside the viewing window in the viewing plane. However, degradation of the window intensity distribution occurs so that the lateral and longitudinal viewing freedom of the observer is reduced compared with that illustrated in FIG. 2. Also, in an ideal display, right eye image data would not be present in the left eye viewing zone and vice versa. However, in practice, crosstalk occurs so that each eye can see some of the light intended for the other eye.
The lateral width of the apertures 7 of the parallax barrier 6 is a compromise between a wide aperture width, which allows high light throughput, and a narrow aperture width, which allows a small geometric image of the source and reduces crosstalk.
The brightness of such a display can be increased by increasing the width of the apertures 7. Also, the parallax barrier structure is visible because the light emission from the display occurs in a relatively small area of the barrier, resulting in a stripy appearance. This appearance may be improved by increasing the aperture width.
Crosstalk causes visual stress to an observer of this type of display. The threshold for crosstalk to avoid this effect is relatively low and may be less than 0.5% depending on the viewing conditions. Crosstalk can be reduced by reducing the aperture width. Similarly, viewing freedom can be increased by reducing the aperture width.
Although some of the performance parameters of the display are not entirely dependent on the width of the apertures 7, such as image brightness which can be changed by changing the intensity of the backlight, the diffraction performance and the crosstalk performance represent a compromise because diffraction performance decreases with decreasing aperture width whereas geometric performance increases. Both diffraction and geometric performance affect blurring of the ideal xe2x80x9ctop hatxe2x80x9d window function.
A known technique for attempting to improve display performance is disclosed in Kirby-Meacham, xe2x80x9cAutostereoscopic displays past and futurexe2x80x9d, SPIE, vol. 624, 1986. This technique involves providing dark columns in the SLM relative to the parallax barrier. However, this technique results in lower display brightness and lower resolution. Also, specially made LCD panels are required which increases cost.
Although the display shown in FIG. 1 is of the front parallax barrier type, rear parallax barrier displays are also used. In such displays, the parallax barrier 6 is disposed on the rear surface of the substrate 2. Rear parallax barrier displays tend to suffer from the effects of Fresnel diffraction because of the diffractive properties of the positioning of a narrow aperture behind a relatively narrow pixel aperture. EP 0 847 208 and GB 2 320 156 disclose a technique for controlling the diffraction effects in a rear parallax barrier display by pre-compensating the illumination of the SLM so as to obtain a more uniform viewing window. EP 0 822 441 and GB 2 315 902 disclose a technique for reducing diffraction from pixel apertures in rear illuminated autostereoscopic displays by varying the pixel aperture function. Also, grey scale modification of the edges of the parallax barrier apertures is disclosed.
Soft edge techniques are also known in other technical fields such as optical systems for astronomy, optical spectroscopy and microscopy. Examples of these techniques are disclosed in G. Toraldo di Francia, Nouvo Cim Suppl vol. 9 p456 (1952); G Boyer et al. Appl vol. 12 p893 (1973); BR Friedon Optica Acta vol. 16 p795 (1969); B Boivin et at. Optica Acta vol. 27 p587 (1980); B Dossier et al. Jour Rech NRS n11 (1950). These techniques relate to the reduction of the Airy disk (zero order) size so as to improve resolution in optical instruments by varying the transmission intensity across light apertures but increasing the brightness of the higher orders of diffraction. A summary of Apodization, where apertures are given a spatially varying transmission function to reduce the size of the Airy disk in coherent precision optical systems such as laser cavities, is disclosed in xe2x80x9cApodizationxe2x80x94coherent optical systemsxe2x80x9d SPIE vol. MS119 (1996) ISBN 0819421502.
According to a first aspect of the invention, there is provided a parallax barrier comprising a plurality of parallel elongate apertures extending in a first direction, characterized in that each of the apertures has an optical transmission function which comprises a plurality of sub-apertures such that the optical transmission function varies in a second direction which is perpendicular to the first direction.
The apertures may be of substantially the same width in the second direction.
The optical transmission function may be substantially constant in the first direction.
At least some of the sub-apertures of each aperture may have different widths in the second direction.
At least some of the sub-apertures of each aperture may have different optical transmission functions in the second direction.
The optical transmission function may be a sinc-squared function in the second direction. As an alternative, the optical transmission function may be the square root of a sum of Gaussian functions in the second direction. As a further alternative, the optical transmission function may be a step function in the second direction. The sub-apertures of each aperture may be superimposed such that the optical transmission function is non-zero throughout the aperture. Each of the apertures may comprise (2m+1) sub-apertures, where m is a positive integer. The optical transmission function in the second direction of the central sub-aperture may have a maximum which is greater than the maxima of the optical transmission functions in the second direction of the other sub-apertures of each aperture, m may be equal to 1.
The barrier may comprise a plurality of layers. The layers of the or each adjacent pair may be substantially in contact with each other. One of the layers may have an optical transmission function which substantially defines the apertures and the or each other of the layers may have an optical transmission function which substantially defines the sub-apertures. The or each other layer may comprise a single other layer. The optical transmission function of the one layer in the second direction may be a substantially rectangular function and the optical transmission function of the other layer in the second direction may be a sinusoidal function superimposed on a constant function so as to have non-zero optical transmission throughout the second direction.
The optical transmission function may be recorded in a photographic medium. As an alternative, the optical transmission function may be encoded as an optic axis orientation function in a birefringent layer cooperating with a polariser. As a further alternative, the barrier may comprise a liquid crystal device having a patterned electrode for applying an electric field across a liquid crystal layer of the device so as to form the optical transmission function.
According to a second aspect of the invention, there is provided an autostereoscopic 3D picture comprising a barrier according to the first aspect of the invention cooperating with a recording medium in which is recorded a spatially multiplexed 3D image.
According to a third aspect of the invention, there is provided an autostereoscopic 3D display comprising a barrier according to the first aspect of the invention cooperating with a spatial light modulator for modulating light with a spatially multiplexed 3D image.
The spatial light modulator may comprise a liquid crystal display.
It is thus possible to provide a parallax barrier of improved performance when associated with other optical devices, for example as part of a 3D picture or display. For example, it is possible to provide a wider range of illuminated area from the surface of a display while maintaining or reducing crosstalk. In particular, improved uniformity of illumination and reduced crosstalk can be obtained compared with conventional parallax barriers with xe2x80x9ctop hatxe2x80x9d optical transmission functions.
A sub-aperture may be defined as an optical transmission function in which there is a maximum or a point of inflexion. In general, each sub-aperture has a maximum in the optical transmission function of the aperture so that the number of sub-apertures is equal to the number of maxima plus the number of points of inflexion.